A fundamental process in development is the specification of embryonic cell fate. Polar lobes, formed by some annelid and mollusc embryos, represent sites of the localization of morphogenetic determinants, an important mechanism for the specification of cell fate. Determinants are shunted to one quadrant of the embryo by these lobes which form transiently during early cleavages. Larvae from lobeless embryos lack lobe- dependent structures including eyes, external shell and statocysts. The role of the Ilyanassa obsoleta polar lobe in development has been studied most extensively, yet many questions remain concerning mechanisms of lobe function and the location and nature of determinants within the lobe. This study proposes to investigate these questions using the following approaches. 1) Equalization of cleavage. When first cleavage is equalized with cytochalasin B or sodium dodecyl sulfate, lobe material is distributed to both blastomeres rather than shunted to one. Eyes and statocysts often are duplicated in resulting larvae; external shell is not duplicated. This analysis will be extended to include separation of equal blastomeres at first cleavage and their rearing as half embryos to gain information about interactions between the two lobe areas in whole equalized embryos. Half embryos also will be analyzed as pairs to see if the single shell observed in whole equalized embryos arises from a double shell gland. 2) Cell lineage. A fate map does not exist for Ilyanassa even though much experimental work would benefit from accurate lineage information. Intracellular tracers such as fluorescent dextrans can be used to assemble lineage information for this embryo. Tracers injected into one or both of the equalized blastomeres and followed will give information about whether double lobe areas act independently in generating lobe-dependent structures or interact in some manner. This approach also may help to determine whether shell arises as a single or double structure in equalized embryos. 3) Cell fusion. Transfer of lobe cytoplasm to a non-lobe blastomere does not alter its development. Since evidence exists suggesting that determinants are bound either to the egg cytoskeleton or plasma membrane, fusion of an isolated polar lobe to a lobeless embryo or blastomere may represent a more reasonable rescue approach, since cytoskeleton and membrane also will be transferred. By fusing isolated lobes onto normal and equalized cells one can examine the effects of altered lobe concentration.